The CAN bus system has become widespread for communication between sensors and control units. In the CAN bus system, messages are transferred using the CAN protocol as described in the CAN specification in ISO 11898. Techniques such as CAN-FD, in which messages can be transferred in accordance with “CAN with Flexible Data Rate, Specification Version 1.0” (source: http://www.semiconductors/bosch.de), etc., have also recently been proposed for this. With such technologies, the maximum possible data rate is increased beyond a value of 1 Mbit/s by using faster cycle timing in the area of the data fields. This can generally be detrimental to transfer quality, for example in the form of a higher bit error rate, when actually existing bus topologies are taken as the basis.
Actually existing bus topologies usually deviate from theory in that reflections occur on the bus line at points at which the bus line exhibits a characteristic impedance deviating from theory. Such points are, for example, branches, incorrect terminations, misadaptation, or pinched wiring, which are often encountered in practical implementations, for example, with stub lines, passive neutral points, etc. The reflections occurring as a result lead to time-related crosstalk of states on the bus line, in such a way that a transmitted symbol or bit “talks over” the symbols following in time, and can distort the sensing thereof.
According to the CAN specification in ISO 11898, the bus line should be terminated at both ends with the line impedance, so that the transient phenomena for the specified maximum cable length decay within a transmitted symbol and an unequivocal state exists at the end of the symbol interval. In reality, however, crosstalk between two or more CAN symbols is unavoidable.
A subscriber station of a CAN bus system is made up of a communication processor that usually is integrated into a microcontroller, and a transmitter/receiver that is also called a “transceiver” and is usually embodied as a separate chip having a direct connection to the bus line. In a transceiver of this kind, the reception path usually encompasses only a comparator preceded by voltage dividers for bias adaptation of the bus level. The comparator directly evaluates the bus level of dominant and recessive bit states and produces a decision at the output.
The transmission path is made up of a respective pull up and pull down transistor for the two bus wires of the CAN bus system, which are also called CAN High (C_H) and CAN Low (C_L) and are used to couple in the dominant level in the transmission state. The recessive levels occur when both transistors are not switched, so that the input resistance of the reception path pulls the levels of the two bus wires of the bus line to an average level. Arbitration of multiple subscriber stations of the bus system can be carried out as a result of different internal resistance levels in the dominant and the recessive state.
The transmission level is established by the switching operations described above. A charge reversal process takes place here because of the predominantly capacitive properties of the bus line of the bus system, with the result that the desired bus level becomes established only after a certain time. The transition from dominant level to recessive level and from recessive level to dominant level takes place at different speeds or time constants.
These different times result in higher error probabilities in the receiving device, which can also be called a “receiver,” because of the slower transition from dominant to recessive bus level. Because an increase in the data rate is achieved in CAN-FD in particular by shortening the symbol duration in the data section, which is transmitted once arbitration is complete, this property could limit the utilization scenarios for the bus system.
The distance between the received signal and the decider threshold is very greatly decreased in this context, so that overlaid noise interference can, with a very much higher probability, result in an incorrect detection in the CAN transmission/reception device.